This invention relates to guidance and control systems and, more particularly, to a method and apparatus for launching, guiding and controlling an object such as a missile toward a target.
There has been a great deal of development with respect to tube-launched, optically-tracked, wire-guided missiles commonly referred to as TOW missiles. On the battlefield, a gunner launches a TOW missile at a target which is, for example, a tank. The missile carries beacons which allow the missile to be located as it moves toward the target. One of these beacons is a long wave length beacon and the other is a short wave length beacon. A gunner typically has a sight, the crosshairs of which he keeps trained on the target to establish a "line-of-sight" therewith. A tracking unit is responsive to the beacons, locations in the field of view (FOV) and to the gunner's aiming point to determine the location of the missile relative to the target. A control system then transmits guidance signals to the missile, via wires running to it, to guide the missile to the target. While initially developed for combat infantrymen to use on the battlefield, the launcher, tracking system and control system can also now be airborne and carried, for example, on a helicopter; or, carried on a ground vehicle. These latter applications allow a greater degree of flexibility, mobility, and survivability for a TOW missile and its crew.
There are a number of problems with respect to tracking and guidance of the missile. One such problem relates to processing the signal developed for the location of the missile in the field of view. When first launched, the missile has an exhaust plume which is sensed as an intense "bow-tie" shaped image superimposed over the image of the long wave length beacon. The tracking signal developed by the tracking unit also registers this intensity level. As the missile moves downrange to the target, the missile's fuel is used up and the plume accordingly disappears. The missile signature is derived from the beacon which travels downrange from the tracker. This results in a diminution of intensity level of the tracking signal. As the missile moves to the target and the energy received from the beacon decreases, the tracking system amplifies the signal before processing it. In some previous TOW missile systems, for example, that shown in U.S. Pat. No. 4,406,429, the signal used for target acquisition and target tracking is also used for missile tracking. However, the requirements of the target tracking system and those of the missile tracking system are not the same and the gain by which the signal is amplified, and the level of the background, do not necessarily have to be the same for both operations. This may therefore require some type of compromise between the target tracking and missile tracking systems. This could effect overall operating efficiency of the system.
A second problem concerns the presence of "blobs" or clutter in the field of view. Blobs appear as points or blooms of light in the display and may be created by countermeasures (CM) taken to present the missile tracker with false targets or to jam the tracking and guidance system. Normal target signature and background clutter may also be sensed on the missile tracking video channel. It is important to be able to quickly and accurately distinguish the missile from the blobs so the missile is not directed away from the target.
A number of schemes have been employed in an attempt to distinguish one object in the field of view from another; and in particular, to track a missile in a CM environment. Among these are edge trackers, centroid trackers, and correlation trackers. In edge tracking, a tracking signal is processed to determine the boundary or edge of the object being tracked. The center or centroid of the object is then determined. In centroid tracking, a tracking signal is processed to determine its center or centroid. In correlation tracking, a reference image of the missile is established. During the course of the missile's flight, a series of scans is made of the field of view which includes the missile. For each scan, the reference image is superimposed over the location in the field of view where the missile is expected to appear. The reference image is then shifted with respect to the field of view until the best mathematical fit or correlation between the reference image and what appears in the field of view is achieved. The resultant location is then considered the location of the missile for that scan.
It will be understood that for each of the above schemes mathematical algorithms are employed to process the data represented by the tracking signal. The algorithms, for example, take into account the diminution of the beacon signal as the missile moves downrange. Also, a defined space or "window" is established within the field of view where the missile is expected to appear and the signal processing is confined to that portion of the signal which represents the area within the window.
One problem with these types of trackers is their inability to distinguish between objects when more than one object appears in the window. Rather, when two or more objects are present, the data for the non-missile objects is "integrated" with that of the missile and the result is a composite that may not represent the true location of the missile. To overcome this difficulty, these systems use a shuttered beacon (so radiation from the beacon can be blocked when the shutter is closed), or an intermittent (blinking) beacon. See, for example, U.S. Pat. Nos. 4,666,103 to Allen, 4,644,397 to Roy et al, and 4,424,943 to Zwirn et al. Each of these patents describes some type of shuttered beacon or blinking beacon system. The premise upon which these systems work is if an object (the missile) appearing at one known time in the field of view can be eliminated from the field of view at another known time, the objects remaining in the field of view at this second known time must be decoys, other missiles, clutter, etc. These objects can then be tagged as such within the tracking system electronics so as to not thereafter interfere with guidance of the missile to its target. While such systems work, difficulties can arise. For example, the shutter system can malfunction. If the shutter system does not block out the beacon when commanded to do so, the point source of light representing the missile will not disappear making it difficult, or impossible, to distinguish the missile from the blobs. Or, if the shutter closes properly, but fails to open, missile tracking is lost. Similar problems occur if a blinking beacon fails in either its "on" or "off" state. The shutter may also be ineffective if the image of the missile and the blob merge. It would be preferable to be able to readily distinguish the missile from the other blobs both without use of a shutter and with a constantly radiating beacon.